Conventional film (or paper) processing devices do not provide a sufficient level of agitation at the film-liquid interface. As a result, a layer of liquid that is depleted of reactants and enriched in reaction by-products exists at the film surface. This layer is the chemical boundary layer. During photographic processing, this boundary layer can influence both the rate at which photochemicals are transferred to and from the film, as well as their concentrations within the film. Either influence can affect the rate and quality of processing. An analogous thermal boundary layer, a region of reduced temperature, is created in the layer of gas which exists at the film surface during the drying portion of the processor.
Analysis and experimental measurements of conventional processors indicate that boundary layers exist which are thick enough so as to become the processing rate limiting parameter. More specifically, the transfer of chemical mass and heat energy through the chemical and thermal boundary layers occurs more slowly than transfer through the film itself. This condition results in low processing speeds, excessively long processing paths and increased size of the processor, including the dryer section. Also, the chemical concentrations in the processor boundary layer need to be excessively high to maintain reasonable mass transport rates resulting in inefficient utilization of the processing chemicals. In the dryer section of the processor drying, temperatures need to be excessively high to maintain reasonable film drying rates resulting in inefficient utilization of thermal energy.
The rate of chemical mass transfer through this boundary layer to the film surface can be approximated to the first degree by the following equation: ##EQU1## where:
m=Mass Transfer Rate (Grams/sec)
A=Film Area (cm.sup.2)
D=Chemical Diffusivity (cm.sup.2 /sec)
.delta.=Boundary Layer Thickness (cm)
.DELTA.C=Concentration Gradient (Grams/CM.sup.3)
The ratio of diffusivity/delta is typically called the mass transfer coefficient. A high mass transfer coefficient will result in a high chemical mass transfer rate, for a given concentration difference between the film and bulk solution concentration. Diffusivity is a function of such variables as molecule size and temperature. While diffusivity can easily be increased by increasing processing solution temperatures (thereby increasing chemical mass transfer rates), this involves the use of complicated equipment and increased cost of processing. Alternatively the chemical mass transfer rate can be increased by decreasing the chemical boundary layer thickness independent of increaseing temperature.
It is known in the prior art that increased agitation decreases boundary layer thickness and that decreased boundary layer thickness produces an increase in developed film density. Also it is known that increased temperatures increase chemical diffusivity. Further, it is known that treating film with a continuous spray (jet impingement) on turbulent flows provides a high degree of agitation, and high reductions in boundary layer thickness. The paper "Heat Transfer Characteristics of Impinging Two Dimensional Air Jets," Gordon and Akfirat, Journal of Heat Transfer, February 1966, pages 101-108, outlines studies of impinging air jets in the heat transfer domain. The results reported indicate that very high heat transfer coefficients can be obtained through the use of direct impinging jets, that the high heat transfer coefficient falls off quite rapidly with increasing distance from the center or stagnation point of the impinging jet, and that the rapid fall off produces an average coefficient over a longer area of impingement that is substantially less than the maximum value.
Typically high turbulent flow rates or jet impingement have been used to achieve adequate mixing. The problem with increasing agitation through turbulence or with jet impingement is that it is extremely difficult to achieve uniformity of the treatment process since any non-laminar fluid condition contains non uniform fluid disturbances such as pockets of turbulence, eddy currents, etc. Also turbulent flows dissipate a great deal of energy in places where it does no useful work, specifically areas away from the film surface. This is due to the high "shearing" action that occurs within the turbulent flow field. Moreover, with turbulent flows and jet impingement it is difficult to treat large areas where uniformity is ever a greater problem.
For the above reasons processors in current use are typically roller transport processors where film or paper is transported by rollers through a tank of solution. While such processors provide more uniform processing than impinging jets or turbulent flows, they are massive in size, inefficient and thus subject to the boundary layer problems discussed above.